This report summarizes several investigations into tissue substitutes for use as aorto-coronary grafts when saphenous veins or internal mammary arteries are unavailable. Three types of tissue conduits have been studied in chronic experiments up to a period of eight months. Adult foxhounds underwent standard sterile cardiac bypass with placement of grafts between the aorta and circumflex coronary artery. Group A consisted of 14 animals who received autologous pericardial tubes. Group B consisted of 6 dogs receiving Spark's mandril grafts. Group C consisted of 20 dogs in which homologous saphenous veins were used in conjunction with low doses of azothioprine. Group A grafts invariably occluded by 3 1/2 months in two patterns: early diffuse thrombosis of the entire graft or late thrombosis of an established graft at distinct sites of graft degeneration, ulceration, and annular narrowing. All Group B grafts clotted within one hour following cardiac bypass. In Group C after adjustment of azothioprine doses, these grafts appear to have an early (3 month) patency rate of 80 percent. These animals will be followed up to a year to evaluate patency. Histologic techniques including Barr body analysis will be used to serially evaluate the degree of host replacement of the donor grafts.